Established human lymphoid cell lines provide uncommonly useful models for the detailed analysis of lymphoid functions of immunologic significance. Our studies have shown that these cells synthesize the putative extracellular mediators of delayed-type hypersensitivity, including migration inhibitory factor (MIF), lymphotoxin, and a specific, reversible inhibitor of lymphoid RNA, protein and DNA synthesis. The remarkable proliferative and synthetic capacities of established human lymphoid cell lines will be utilized to continuously supply in large amounts the raw materials required for isolation and purification of these factors. Materials purified by ultrafiltration, column chromotography, and polyacrylamide gel electrophoresis will be used for comparative study of their physico-chemical and biologic characteristics. Collateral study of similar products from phytomi togen and antigen stimulated human peripheral lymphocytes and from human non-lymphoid cell lines will help determine the specific "immunologic" role of these mediators. With purified materials we will develop specific reagents to detect by immunologic means the identity, production, sites of attachment and mechanisms of action of these factors.